America's top intelligence officers poring over new images of a suicide bombing in Syria unlike any other. Because they believe one of those suicide bombers was an American citizen a man from Florida who is he and have other Americans been turned into killers to. ABC senior justice correspondent Pierre Thomas says those new developments tonight. It was a powerful truck bomb in northern Syria. The al-Qaeda affiliated terrorists who produced this video of the attack say this American was a suicide bomber who drove the truck the video shows the truck being loaded with -- And driving toward its target. Government sources say the cat -- suspect is a US citizen from Florida. The first American to carry out a suicide attack in Syria. US officials are reaching out to the man's family and are investigating how he was recruited American fighters in Syria. A potentially imminent terror threat sparking concerns at the highest reaches of government adversary -- Civil war spilled across borders the capacity of battle hardened. Extremist groups to come after us only increase. At least seventy Americans are believed to a -- the thousands of foreign fighters is -- some have made their way back to the US sources say. And are under investigation -- surveillance and ABC news exclusive an FBI director James -- recently warned that America's who've gone to fight in the Syrian conflict. Might return home to formed sleeper cell for al-Qaeda. Can -- give us a sense of how active you are making sure. That you keep an eye on those people we are very active in civil war there is offering an opportunity for thousands. A foreign fighters to get training in the worst parts of terrorism they're gonna come back to Europe into the United States. A civil war in a land far away with potentially huge implications. For the US homeless. Pierre Thomas ABC news Washington.

This transcript has been automatically generated and may not be 100% accurate.

Now Playing: Suicide Bomber Kills 100 in Yemen

Now Playing: Deadly terror attack on streets of Barcelona

Now Playing: Two people in custody in connection with Barcelona attack

Now Playing: Trump rails against the removal of 'beautiful statues'

Now Playing: Millions of people on severe weather alerts from Michigan to Pennsylvania

Now Playing: Hurricane Gert may cause rip currents from New Jersey to Maine

Now Playing: Crew members on Jet Blue flights sickened

Now Playing: More details emerge about the man charged with Heather Heyer's murder

Now Playing: Charlottesville holds memorial service for woman killed in car ramming

Now Playing: Donald Trump's news conference spirals out of control

Now Playing: Chad Bettis returns to the mound

Now Playing: {{itm.title}}

{"id":23921054,"title":"Rising Threat: American Suicide Bombers","duration":"1:53","description":"Why are Americans turning to heinous and deadly acts of terror?","url":"/WNT/video/rising-threat-american-suicide-bombers-23921054","section":"WNT","mediaType":"default"}